Maintenance of EHV Substation - scopes - 2016

International conference on Signal Processing, Communication, Power and Embedded System (SCOPES)-2016

Maintenance of EHV Substation Mohammed abdul khader aziz biabani

Syed rehan mohsin ahmed

Research Scholar [Power electronics & system] EED Dept Muffakham Jah College of Engineering & Technology Hyderabad, India [email protected]

Research student [Power systems] EED Dept Muffakham Jah College of Engineering & Technology Hyderabad, India [email protected]

Abstract— Substation maintenance is a process of periodic, planned inspection of and, if necessary, repair, and replacement of all switchgear, buildings, and ancillary equipment in substation installations. In simple terms, substation maintenance is a regimen of regular preventative checks and actions carried out to ensure substations are kept in good working order. This process typically consists of a series of stringent visual and physical inspections and actions carried out according to a set schedule. All inspection and actions during substation maintenance should be accurately documented and stored for further reference. Substations are the most critical part of any electrical supply grid. Substation care is one of the most important parts of any electrical department’s preventative maintenance schedule. These schedules generally consist of a tiered plan of minor and major maintenance events. Minor maintenance will seldom require any sort of power shut down although major events typically requiring short intervals of supply interruption. Routine or minor substation maintenance is usually carried out on a weekly or bi-weekly basis. It typically consists of visual checks and superficial cleaning only. Major substation maintenance involves more planning and is generally handled on a quarterly basis. In the case of industrial installations, these procedures are often carried out during plant maintenance shutdowns. Ancillary equipment such as amp and volt meters, control circuits, and transformers will also be given individual attention during this review. Keywords—Circuit breakers, Current transformer, Isolator, Lightning arrestors, Capacitor voltage transformer, Wave trap, Potential transformer, Power transformer, Station transformer, Busbars, Capacitor bank, Control&Relay panel, Station Battery charger, Station charger, AC distribution panel, Air-conditioning, Firefighting equipment and Diesel generator set.

I.

INTRODUCTION TO ELECTRICAL SUBSTATION

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Electric power may flow through several substations between generating plant and consumer, and its voltage may change in several steps. Substations may be owned and operated by a transmission or generation electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are

978-1-5090-4620-1/16/$31.00 ©2016 IEEE.

un-attended, relying on SCADA for remote supervision and control.

Fig.1 Typical sub-station layout A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station. A. Elements and Functions of sub-station Elements: Substations generally have switching, protection and control equipment, and transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distributions stations may use reclose circuit breakers or fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation. Functions:  Supply electric power to the consumers continuously  Supply of electric power within specified voltage limits and frequency limits  Shortest possible fault duration.  Optimum efficiency of plants and the network  Supply of electrical energy to the consumers at lowest cost

B. Types of Sub-Stations Sub-stations are broadly classified on the nature of their nature of duties, service rendered, operation voltage, sub-station design. Based ON Nature Of Duties:  Step up or primary Electrical Power substation.  Primary Grid Electrical Power Substation.  Step Down or Distribution Electrical Power Substations. Basis Of Service Rendered:  Transformer Substation  Switching Substation  Converting Substation Based on Operation Voltage:  High Voltage Electrical Power Substation (operating voltage between 11kv-66kv). II.

established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a "short circuit"). B. Equipments used in EHV sub-station Substation has different equipments which are used for different functions; they are used in changing voltage and equipments for the protection of substations which protect the substation from the faults which occur. Following are the different equipments used in EHV substation.

REQUIREMENTS IN EHV SUB-STATION

A. Design of sub-staion The main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow expansion of the station, when required. Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects. A grounding (earthing) system must be designed, and ground potential rise must be calculated to protect passers-by during a short-circuit in the transmission system. The substation site must be reasonably central to the distribution area to be served The first step in planning a substation layout is the preparation of a one-line diagram which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. In a common design, incoming lines have a disconnect switch and a circuit breaker. In some cases, the lines will not have both, with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off, or to cut off a line when power is flowing in the 'wrong' direction. When a large fault current flows through the circuit breaker, this is detected through the use of current transformers. The magnitude of the current transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center. The arrangement of switches, circuit breakers and buses used affects the cost and reliability of the substation .Once having

Fig.2 A: Primary power lines' side B: Secondary power lines' side 1.Primary power lines 2.Ground wire 3.Overhead lines 4.Transformer for measurement of electric voltage 5.Disconnect switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester 9.Main transformer 10.Control building 11.Security fence 12.Secondary power lines. C. Circuit Breaker A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.

(a) (b) Fig.3 (a) 115 kV bulk oil circuit breakers (b) 400 kV SF6 live tank circuit breakers Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electro technical Commission (IEC). High-voltage breakers are nearly always solenoidoperated, with current sensing protective relays operated through current transformers. In substations the protective

relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc.  Bulk oil  Minimum oil  Air blast  Vacuum  SF6 (SULFUR HEXAFLUORIDE) Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc. SULFUR HEXAFLUORIDE (SF6) CIRCUIT-BREAKERS: A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. D. Current Transformer In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current. E. Isolators Isolator is an equipment which is used to make or break the circuit in electrical transmission, a disconnect or isolator switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair.

Fig.5 Isolator F. Lightning Arrestors A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a highvoltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

Fig.6 Lightning Arresters (a) (b) Fig.4 (a) HV current transformer (b) Current transformers used in metering equipment for three-phase 400 ampere electricity supply. Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary

G. Capacitor Voltage Transformer A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or protective relay.

Fig.9 Power Transformer Fig.7 Capacitor Voltage Transformer H. Potential Transformer Potential transformers are instrument transformers. They have a large number of primary turns and a few number of secondary turns. It is used to control the large value of voltage. The potential transformer works along the same principle of other transformers. It converts voltages from high to low. It will take the thousands of volts behind power transmission systems and step the voltage down to something that meters can handle. These transformers work for single and three phase systems, and are attached at a point where it is convenient to measure the voltage. It is used to measure the voltage of the bus.

J.

Station Transformer In general station service transformer is used for supplying power to auxiliary equipment in the power plant when the plant is not generating any power.  Rated HV voltage corresponds to the rated voltage of the outer bus bars  Rated LV voltage corresponds to the auxiliary bus voltage  Rated KVA corresponds to the load of common auxiliaries of the station. This corresponds to the 10% to 15% of the rating of the generating power. These transformers are Outdoor type.

Fig. 8 Potential transformer I.

Power Transformer Power Transformer is important equipment in substation. A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling. By appropriate selection of the ratio of turns, a transformer thus enables an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np. The windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. There are different types of transformers they are  Step-Up Transmission Substations  Step-Down Transmission Substation  Distribution Substation

Fig.10 Station Transformer K. Busbars(Conductors/Pipes) In electrical power distribution, a bus bar is a strip of copper or aluminum that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. The size of the bus bar determines the maximum amount of current that can be safely carried. Bus bars can have a cross-sectional area of as little as 10 mm2 but electrical substations may use metal tubes of 50 mm in diameter (1,963 mm2) or more as bus bars. An aluminum smelter will have very large bus bars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminum from molten salts. Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. A bus bar may either be supported on insulators, or else insulation may completely surround it. Bus bars are protected

from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly onto any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in the form of bus duct or bus way, segregatedphase bus, or isolated-phase bus.  For132kv (main bus)-ACSR Zebra.  For132kv (auxiliary bus)-ACSR Panther.  For 33kv we use ACSR Zebra for both main and auxiliary bus bars.

function of OLTC is to give constant voltage to Secondary winding.

Fig. 13 OLTC Transformer panel

(a) (b) Fig.11 (a) ACSR PANTER (b) ACSR ZEBRA L.

Capacitor Bank A capacitor bank is a grouping of several identical capacitors interconnected in parallel or in series with one another. These groups of capacitors are typically used to correct or counteract undesirable characteristics, such as power factor lag or phase shifts inherent in alternating current (AC) electrical power supplies. Capacitor banks may also be used in direct current (DC) power supplies to increase stored energy and improve the ripple current capacity of the power supply.

N. Station battery charger and station battery The operation of monitoring devices such as relays and the tripping mechanisms of breakers require an independent power source, which does not vary with main source being monitored. Batteries provide this power source for the successful operation of switching and control devices in power protection system. It is therefore necessary to ensure that batteries and charges are regularly inspected and maintained at the highest possible level of efficiency at all times to enable correct operation of relays at the correct time. These batteries are charged by station battery charges.

Fig.14 Station batteries O. AC Distribution Panel It is a part of control room, were all the control of the substation is done.

Fig.12 Capacitor bank in a substation M. Control & Relay panel/OLTC Panel OLTC means on load tap changer. The location of OLTC in Transformer is in HV winding. The tapping is taken from HV winding because in HV winding number Of turns are more. So, voltage regulation is good. The HV winding is nearer to the core. So, the tapping is Possible. In HV winding current carrying capacity is less than LV winding. The

Fig. 15 Sub-station control room

III.

SUBSTATION MAINTENANCE

The activity carried out to prevent the occurrence of failure, the activity carried out after failure to rectify the defect in substation equipment’s is called substation Maintenance. It is very much useful to increase the operational reliability of the substation equipment’s. Failures are inevitable due to wear and tear of the mechanical and electrical equipment’s. This can be minimized by constant maintenance of the substation equipments. A. Objective of maintenance  The primary objective of maintenance is to increase the operational reliability and personnel safety at minimum cost.  The main objective of substation maintenance is to avoid the breakdown of the transmission system due to wear and tear of the electrical equipments. B. Service life Electrical equipment designed for certain economic service life Equipment life dependent on operating environment, maintenance program, quality of the original manufacture and installation Beyond service life period they are not expected to render their services according to its expectations with desired efficiency General causes of equipment failure much earlier than expected economic life span? Poor quality of raw material, Workmanship and manufacturing techniques, Frequent electrical, mechanical, and thermal stresses during the operation. C. Types of maintenance

Fig.16 Classification of Substation Maintenance 1) Preventive maintenance: The Activity which is carried out to prevent the occurrence of failure is known as preventive maintenance. It is the action performed to prevent failure by providing systematic inspection and monitoring to detect and prevent incipient deterioration or failure and includes testing to confirm correct operation.

Corrective maintenance work performed as a result of significant deterioration or failure, to restore an asset to its required condition standard. The work may be programmed as a result of condition assessment or as random additions to the program based on priority. 2) Preventive Maintenance Is Further Divided Into:  Direct Preventive Maintenance  In direct Preventive Maintenance a) Direct Preventive Maintenance:  Direct Preventive maintenance activities such as cleaning, lubricating, replacement of wear/tear parts and overhauling of equipment.  Most of the common failures which are likely to occur during in service of the equipment can be arrested.  This cannot give 100% operational reliability, because there is always chance of unexpected failures to occur. b) In-Direct Preventive Maintenance:  By constantly monitoring the observable characteristics any major faults /failures that is being gradually developed can be detected beforehand.  This constant monitoring of observable characteristics and measurable parameters in any equipment is called “condition monitoring”.  In-directive preventive maintenance helps to take corrective action in planned manner by giving the reports as and when parameter changes sharply. 3) The Following Maintenance Activities Are To Be Carried Out In preventive Maintenance:  Daily Maintenance- Maintenance Carried out Daily  Weekly maintenance- Maintenance Carried out Every Week  Monthly Maintenance-Maintenance Carried out Per Month.  Quarterly Maintenance- Maintenance Carried For Every Three Months.  Half Yearly Maintenance- Maintenance Carried out For Every Six Months.  Yearly Maintenance-Maintenance Carried out Yearly. 4) Break down/ Corrective maintenance: The maintenance which is carried out after the occurrence of failures is called as break down maintenance to corrective failure is called as corrective maintenance. BREAK DOWN maintenance actions performed to restore an asset to operational condition, as a result of an unforeseen failure. Unplanned maintenance actions to restore. IV.

MAINTENANCE OF EQUIPMENTS IN EHV SUBSTATION

A. EQUIPMENTS FOR MAITENANCE: The following are the equipments in substation that are maintained.  Circuit Breaker

 Current Transformer  Isolator (with/without earth switch)  Lightning Arresters  Capacitor voltage Transformer  Wave trap  Potential Transformer  Power Transformer  Station transformer  Bus bars (conductor/pipe)  Capacitor Bank  Control & relay panel/ OLTC panel  Station Battery charger  Station Battery  AC distribution panel  Air-conditioning.  Firefighting equipment.  Diesel Generator set. General Maintenance Is Done For The Following:  Control Room  Switch yard premises. B. PREVENTIVE MAINTENANCE OF EQUIPMENTS: a) Circuit Breaker: Daily Maintenance: Draining of moisture from all air compressors Checking of SF6 gas pressure Checking of Pneumatic /Hydraulic pressure and leakages Check any red hot on power connectors. Check any abnormalities. Weekly Maintenance: Running hours of compressor/Hydraulic pump Check oil levels in compressor/Hydraulic drive. Check any abnormalities. Monthly Maintenance: Check oil leakages in compressor/Hydraulic drive if any. Check oil levels in compressor/Hydraulic drive, top up if required. Checking oil/hydraulic pressures i.e Pump ON: ______bar Pump OFF: _______bar Check any abnormalities. Quarterly Maintenance: Apply lubrication on all moving parts ( oil/Grease) Clean Breaker Mechanism/marshaling box. Check and maintenance of illumination and space heater in MB Check any abnormalities. Half yearly Maintenance: Functional checking of Air compressor/ Hydraulic motor auto starting Clean Breaker Mechanism/marshaling box. Check and maintenance of illumination and space heater in MB Check any abnormalities. Yearly Maintenance:

For SF6 CB check SF6 gas leak test (to be done on complete CB including interrupter chamber) Check pressure drop during duty cycle. b) Before BKR open Oil pressure/ Air pressure Pre coming----------/----------Actual ----------/----------c) After BKR open Oil pressure/ Air pressure Pre coming----------/----------Actual ----------/----------d) Before BKR Close Oil pressure/ Air pressure Pre coming----------/----------Actual ----------/----------e) Before BKR Close Oil pressure/ Air pressure Pre coming----------/----------Actual ----------/---------Checking of hydraulic pump operation Start pressure. Stop pressure. Checking of Breaker timings (pre commission/Actual) 1) BKR closing time. 2) BKR tripping time for coil: 1 & 2. 3) Close/Trip time pole discrepancy. 4) Break to break of same phase. Checking of Inter locks. Tightening of all control cable terminal connections & earth connections of BKR/MB Tightening of all BKR power connections. Cleaning of BKR mechanism and MB Lubrication of all moving parts in BKR mechanism Lubrication of BKR mechanism box / MB door hinges Checking of Door sealing gas cuts and replacement thereof if necessary Cleaning of breaker poles Checking of operational lock out Driving Mechanism (Hydraulic/Pneumatic) for set value/actual value Checking of healthiness of operation counter Measurement of contact resistance Resistance between the fixed and moving contacts of poles during closed condition. The measured resistance is acceptable if it is less than 150 micro ohms Measurement of IR values (Megger). Resistance between the fixed and moving contacts of poles during closed / open condition. 1) The measured resistance is giga ohms if the BKR is in OFF condition. 2) The measured resistance is nearly Zero ohms if the BKR is in ON condition. 3) Measurement of IR values of control cables. b) Current Transformer: Daily Maintenance: Checking of oil levels in the inspection glass,

Checking of oil leakages, Check any red hot on power connectors. Check for any abnormal sound other than usual corona noise. Quarterly Maintenance: Checking of gas cuts healthiness replace if it requires, Checking of any oil leakages from secondary terminal box, Cleaning of CTMB, Tightening of Power Connectors/Earthing. Yearly Maintenance: Measurement of IR Values 1) Primary to earth… (Min: 1000 M ohm) by 5KV Megger 2) Primary to Sec…… (Min: 1000 M ohm) by 0.5 KV Megger 3) Secondary by 0.5 KV Megger. Core 1,2,3,4 &5 to earth: …..( Min: 50 M ohm) Measurement of Tan Delta & Capacitance Values Tan delta value Capacitance value Checking (Tightening) the power connectors and secondary terminal connectors (at CT and CT MB) Cleaning of CT and visual inspection for any abnormalities CT ratio test (by Primary injection) During primary injection measure the actual Primary & Secondary currents and compare with the Pre commissioning values. Measurement of control cable IR value c) Isolator (With/Without Earth Switch): Daily Maintenance: Check any red hot on power connectors. Check for any cracks on Solid core Insulator. Yearly Maintenance: Operating Mechanism 1) Checking of linkages including transmission gears. 2) Checking of stopper bolts & tightening. 3) Checking of Aux switch contacts & tightening of terminals. 4) Lubrication of all moving parts. 5) Checking of Mechanical/Electrical interlocks. 6) Checking & tightening of all bolts & nuts. Main Contacts: 1) Checking &Lubrication of Main contacts. 2) Checking of Alignment for opening / closing. 3) Tightening of bolts &nuts, pins. 4) Checking of Mechanical/Electrical interlocks. 5) Cleaning of support Insulators. 6) Measurement of main contact resistance, measured value is less than 300 micro ohms. Earth Switch: 1) Checking &Lubrication of Main contacts. 2) Checking and Alignment of earth blades. 3) Tightening of bolts &nuts, pins. 4) Checking operation of earth switch & Mechanical interlocks. Marshaling box: 1) Cleaning & tightening of auxiliary contact terminals. 2) Checking on Motor operation & connected circuit IR Value Measurement by 5.0 KV megger: 1) Top stack to earth, measured value is to be minimum 1000 Mega ohm.

d) Lightning Arrestor: Daily Maintenance: Checking of counter reading (it indicates the LA operations) Checking surge monitor current (Green/Red zone), it shows the healthiness of the LA. In service condition surge monitor current to be in green zone only, if shows Red zone the LA to be replaced with new LA otherwise it may lead to failure. Yearly Maintenance: Testing of counter, Testing of Surge monitor. IR Value measurement with 5.0 KV megger, Top stack to earth, the measured value is to be greater than 1000 mega ohm. e) Capacitor Voltage Transformer: Daily / Weekly Maintenance: Check for oil level & oil leakages. Check for Abnormal sound other than usual corona noise. Monthly Maintenance: Checking of oil leaks & healthiness of rubber gas cuts, Visual checking of earthing of HF point, Checking of any cracks/ breakage in HF bushing, Measurement of voltages in MB a) Core 1,2 &3 phase to neutral and analyses the voltage ratio error. b) Error is: +/-5% for Protection core. c) Error is: +/-0.5% for Metering core. Yearly Maintenance: Checking of power connectors & tightening of secondary terminal connections. Checking of oil level in the inspection glass, if it is low replace the CVT, Measurement of secondary cable resistance (After removing CVT sec wires) Tan delta & capacitance value measurement The permissible limit for capacitance value: +10% / -5% of the rated value, Checking of neutral earthing in CVT MB, Cleaning of marshaling box & junction box f) Wave Trap: Daily Maintenance: Visual observation of power connectors & any cracks, Yearly Maintenance: Checking for any cracks & tightening of power connectors. g) Potential Transformer: Daily Maintenance: Checking of oil levels in the inspection glass, Checking of oil leakages, Check any red hot on power connectors. Check for any abnormal sound other than usual corona noise. Quarterly Maintenance: Checking of gas cuts healthiness replace if it requires. Checking of any oil leakages from secondary terminal box, Cleaning of PTMB, Yearly Maintenance: Measurement of IR Values, a) Primary to earth… (Min: 1000 M ohm) by 5KV Megger

Primary to Sec…...(Min: 1000 M ohm) by 0.5 KV Megger b) Secondary by 0.5 KV Megger. 1) Core 1, 2&3 to earth: ….. (Min: 50 M ohm) 2) Measurement of Tan -Delta & Capacitance Values, Tan delta value. The "tan delta test" is a diagnostic procedure to assess the deterioration of the insulation of equipment. (UST Mode: Ungrounded Specimen Test mode) (GST Mode: Grounded Specimen Test mode). Capacitance Measurement Checking (Tightening) the power connectors and secondary terminal connectors (at PT and PT MB), Cleaning of PT and visual inspection for any abnormalities, PT ratio test is to be measured, Measurement of control cable IR value h) Power Transformer: Daily Maintenance: Checking of oil level in conservator/OLTC /Bushing, Checking winding/oil temp indicator, Checking of cooling fan condition, Checking of oil leakages from bushing /entire PTR, Check for unusual internal noise, Checking for relief diaphragm for cracks, Check oil level in breather oil cups &fill.

Fig. 17 Cleaning of Power transformer Monthly Maintenance: Checking of Conservator/OLTC/ bushing oil level, Checking of auto starting of cooler fans/ pump, Checking of oil leakages, Checking of any abnormalities, Checking the color of silica gel in the breather, replace if the color changes from blue to pink, inspect for any cracks & clean, Ensure that the oil comes out when air release valve is opened, if the provision is available at bottom of the PTR, Checking the oil level in inspection glass of Buchholz relay, Measure the transformer earth pit resistance value. Quarterly Maintenance: Checking of oil level in conservator/OLTC /Bushing, Checking of auto starting of cooler fans/ pump, Checking oil leakages & arresting oil leaks. Measurement of Insulation resistance with 5 KV Megger a) PTR HV to Body-------ohms b) PTR LV to Body-------ohms c) PTR HV to LV -------ohms.

Check oil levels in inspection glass of Buchholz relay ensure that oil comes out when air release valve is opened Inspection of bushings for any cracks, chipping out & cleaning of bushings Checking & tightening of all power connections / control cable termination of PTR Checking of Buchholz relay for any gas collection passing through silver nitrate solution Checking PTR ground connection for tightness Half Yearly Maintenance: Testing of Top/Bottom oil sample for DGA test, BDV Checking up of gap setting of the bushing on the PTR if available Inspection of bushings for any cracks, chipping out & cleaning of bushings Checking of oil level in conservator/OLTC /Bushing Checking of auto starting of cooler fans/ pump Cleaning and tightening of FCC & OLTC box & RTCC Measurement of Insulation resistance with 5 KV Megger(note temp). a) PTR HV to Body-------ohms b) PTR LV to Body-------ohms c) PTR HV to LV -------ohms. Yearly Maintenance: Measurement of Insulation resistance with 5 KV Megger( note temp). a) PTR HV to Body-------ohms b) PTR LV to Body-------ohms c) PTR HV to LV -------ohms. Calibration of temp indicators (winding/oil) & checking functioning of RWTI & ROTI Checking operation of Buchholz relay by air injection, Inspection of bushings for any cracks, chipping out & cleaning of bushings, Checking of auto starting of cooler fans/ pump, Checking of gas cuts healthiness replace if it requires. Checking of any oil leakages from secondary terminal box, Cleaning of CTMB, Tightening of Power connectors/Earthing i) Station Transformer: Daily Maintenance: Checking of oil level in the conservator tank, Checking of oil leakages from bushing / entire Station Transformer, Checking of Secondary voltages (ph to ph&ph to Neutral), Checking for any abnormal noise Monthly Maintenance: Checking of Conservator/ Bushing oil level, Checking of oil leakages, Checking of any abnormalities Checking the color of silica gel in the breather, replace if the color changes from blue to pink, inspect for any cracks & clean. Yearly Maintenance: Measurement of Insulation resistance with 5 KV Megger (note temp) a) PTR HV to Body-------ohms.

b) PTR LV to Body-------ohms. c) PTR HV to LV -------ohms. Checking of oil leakages & tightening. Checking of Secondary voltages (ph to ph&ph to Neutral) Tightening of all power connections/terminal connections j) Bus Bars: Daily Maintenance: Checking of red hot Checking of any strands cut Quarterly Maintenance: Thermo vision Scanning on all connectors. Rectification is to be carried out if the temp exceeds more than 60 degree centigrade. Yearly Maintenance: Thermo vision Scanning on all connectors. Rectification is to be carried out if the temp exceeds more than 60 degree centigrade Checking of Insulators for cracks Cleaning of Insulators Measurement of Earth resistance

Fig. 18 Thermo vision scanning of connectors k) Capacitor Bank: A set of Capacitor bank includes Breaker, CT, RVT, Capacitor cells. Maintenance of Breaker, CT is same as above. Capacitor Cells: Daily Maintenance: Checking of any oil leakages Control & Relay Panel: Yearly Maintenance: Cleaning of relays/panel Tightening of all terminals Station Battery Charger: Daily Maintenance: Checking of healthiness of 3 ph AC supply Checking of fault alarm, Indications Checking of Auto/ manual mode Checking of DC Float voltage/ Current Checking of DC earth leakage Monthly Maintenance: Cleaning of battery charger Checking of fault alarm, Indications Checking of Auto/ manual mode Checking of DC Float voltage/ Currents before and after battery maintenance Checking of DC earth leakage

Checking of Relay contacts Tightening of terminal connections l) Station Battery: Daily Maintenance: Checking & cleaning of battery terminals and application of petroleum jelly. Checking of Acid levels in lead acid battery & filling. Checking of any acid leakages / abnormalities in battery Checking condition of Exhaust fan Monthly Maintenance: Checking & cleaning of battery terminals and application of petroleum jelly. Checking of Acid levels in lead acid battery & filling Checking of any acid leakages /abnormalities in battery Checking condition of Exhaust fan Measurement of Individual cell Specific gravity & Voltages. Cell no. Voltage Specific Gravity After measuring the Individual battery voltage/Specific gravity, batteries to be charged in boost mode. The following Important observation is to be recorded in the battery. a) Before charger OFF: ---V-----Amp b) After charger OFF : ---V-----Amp c) Before Charger ON: ---V-----Amp The above observation indicates the healthiness of Battery. m) AC Distribution Board Daily Maintenance: Checking Input AC Voltage i.e. phase to phase and phase to neutral voltages. Yearly maintenance: Check condition of cable, MCB, fuses, switches etc. Cleaning of panel Tightening of cable terminations n) Diesel Generator Set: Daily Maintenance: Check Engine oil level & leakage. Check the fuel oil level (stock) & leakage. Weekly Maintenance: Clean complete system. Check Engine oil condition. Run the DG set for 30 Minutes at no load and check the following. Check Voltage (i.e. ph. to ph. & ph. to neutral), frequency o) Break-down maintenance of the equipments: Break down maintenance actions performed to restore an asset to operational condition, as a result of an unforeseen failure. Necessary maintenance actions are taken in order to regain the power circuit in the faulty conditions break down of the substation equipments. In breakdown of the equipments it is vital to restore the fault with a very less time. p) General maintenance: General maintenance like cleaning and general protection is done for the following. 1) Control Room 2) Switch Yard Premises

Fig. 19 Control room

Fig. 20 Switch yard premises V.

PROTECTION OF SUBSTATION EQUIPMENTS

A. Need for protection of substation equipments: Substation equipments are the important components which constitutes the electrical substation. Any fault in any of the equipments result in the breakdown of the substation. So there is a very essential need to have a necessary precautions effort to protect the substation equipments. B. Components used for the substation protection: The important components which protect the substation equipments form fault conditions are: 1) Protective relay 2) Circuit breaker C. Protective relay as the substation protector: A protective relay is an electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected. Unlike switching type relays with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays have well-established, selectable, time/current (or other operating parameter) operating characteristics. Protection relays may use arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay contacts, and phase-shifting networks. Protection relays respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency. Distance relays trip for faults up to a certain distance away from a substation but not beyond

that point. An important transmission line or generator unit will have cubicles dedicated to protection, with many individual electromechanical devices. D. Operation principles of relay: Electromechanical protective relays operate by either magnetic attraction, or magnetic induction. “Armature"-type relays have a pivoted lever supported on a hinge or knife-edge pivot, which carries a moving contact. These relays may work on either alternating or direct current, but for alternating current, a shading coil on the pole is used to maintain contact force throughout the alternating current cycle. Because the air gap between the fixed coil and the moving armature becomes much smaller when the relay has operated, the current required to maintain the relay closed is much smaller than the current to first operate it. The "returning ratio" or "differential" is the measure of how much the current must be reduced to reset the relay. E. Digital protective relays: The functions of electromechanical protection systems are now being replaced by microprocessor-based digital protective relays, sometimes called "numeric relays". These convert voltage and currents to digital form and process the resulting measurements using a microprocessor. The digital relay can emulate functions of many discrete electromechanical relays in one device, simplifying protection design and maintenance. Each digital relay can run self-test routines to confirm its readiness and alarm if a fault is detected. Numeric relays can also provide functions such as communications (SCADA) interface, monitoring of contact inputs, metering, waveform analysis, and other useful features. Digital relays can, for example, store two sets of protection parameters, which allow the behavior of the relay to be changed during maintenance of attached equipment.

Fig. 21 5.0A Microprocessor-Based digital protection relay can replace the functions of many discrete electromechanical instruments

trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

Fig. 22 Relay and Control Panel in Substation F. Circuit breakers: A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. G. High-Voltage Circuit Breakers: Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electro technical Commission (IEC). High-voltage breakers are nearly always solenoidoperated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc.  Bulk oil  Minimum oil  Air blast  Vacuum  SF6 Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc. Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1200kV breakers were launched by Siemens in November 2011. High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to

(a) (b) Fig. 23 (a) Oil circuit breaker (b) SF6 circuit breaker VI. CONCLUSION Finally we can conclude that maintenance of substation is very much useful in avoiding the future failure and breakdown of substation and substation equipments. A planned maintenance of substation will have many uses, they can save money and they can also use to avoid the wear and tear of the equipments and also time of the men. As we know that electricity is the main source of energy for the people , disturbance in power supply will cause many problems so maintenance of substation equipments is very essential. Primary equipments in a substation play a very important role in maintaining power system integrity and power supply availability to the end user. Modern power system is very complex in nature which produces Transients during various switching operations, stability problems at different operating contingencies and power system faults, instrument transformers non-linear response causing secondary equipment misoperation resulting further degradation of power system integrity. Power system elements like transformers and transformer bushings can damage due to various reasons and installing online condition monitoring /Power quality monitoring system can provide early warning message to take immediate corrective action. Flexible AC Transmission System (FACTS) provide system stability enhancement. The new technologies provide a face-lift to the described problem. Such Technological changes are applicable to Power Transformers, Circuit Breakers and Instrument Transformers. REFERENCES [1] G.N. Mathur, “Manual on EHV Substation Equipment Maintenance.” CENTRAL BOARD OF IRRIGATION AND POWER. [2] John D. McDonald “Electric Power Substation Engineering.”CRC PRESS. [3] S.Rao “Electrical Substation Engineering & Practice: EHV-HVDC & SFGIS (Principle, Practice, Design and Reference Data).”KHANNA PUBLISHERS.